gobject: Add missing (nullable) and (out) annotations on the data/qdata API

https://bugzilla.gnome.org/show_bug.cgi?id=756588

gobject/gobject.c

@@ -3375,7 +3375,7 @@ g_clear_object (volatile GObject **object_ptr)
  * This function gets back user data pointers stored via
  * g_object_set_qdata().
  * 
- * Returns: (transfer none): The user data pointer set, or %NULL
+ * Returns: (transfer none) (nullable): The user data pointer set, or %NULL
  */
 gpointer
 g_object_get_qdata (GObject *object,
@@ -3390,7 +3390,7 @@ g_object_get_qdata (GObject *object,
  * g_object_set_qdata: (skip)
  * @object: The GObject to set store a user data pointer
  * @quark: A #GQuark, naming the user data pointer
- * @data: An opaque user data pointer
+ * @data: (nullable): An opaque user data pointer
  *
  * This sets an opaque, named pointer on an object.
  * The name is specified through a #GQuark (retrived e.g. via
@@ -3460,7 +3460,7 @@ g_object_dup_qdata (GObject        *object,
  * @oldval: (nullable): the old value to compare against
  * @newval: (nullable): the new value
  * @destroy: (nullable): a destroy notify for the new value
- * @old_destroy: (nullable): destroy notify for the existing value
+ * @old_destroy: (out) (optional): destroy notify for the existing value
  *
  * Compares the user data for the key @quark on @object with
  * @oldval, and if they are the same, replaces @oldval with
@@ -3501,8 +3501,8 @@ g_object_replace_qdata (GObject        *object,
  * g_object_set_qdata_full: (skip)
  * @object: The GObject to set store a user data pointer
  * @quark: A #GQuark, naming the user data pointer
- * @data: An opaque user data pointer
+ * @data: (nullable): An opaque user data pointer
- * @destroy: Function to invoke with @data as argument, when @data
+ * @destroy: (nullable): Function to invoke with @data as argument, when @data
  *           needs to be freed
  *
  * This function works like g_object_set_qdata(), but in addition,
@@ -3565,7 +3565,7 @@ g_object_set_qdata_full (GObject       *object,
  * and thus the partial string list would have been freed upon
  * g_object_set_qdata_full().
  *
- * Returns: (transfer full): The user data pointer set, or %NULL
+ * Returns: (transfer full) (nullable): The user data pointer set, or %NULL
  */
 gpointer
 g_object_steal_qdata (GObject *object,
@@ -3584,7 +3584,8 @@ g_object_steal_qdata (GObject *object,
  * 
  * Gets a named field from the objects table of associations (see g_object_set_data()).
  * 
- * Returns: (transfer none): the data if found, or %NULL if no such data exists.
+ * Returns: (transfer none) (nullable): the data if found,
+ *          or %NULL if no such data exists.
  */
 gpointer
 g_object_get_data (GObject     *object,
@@ -3600,7 +3601,7 @@ g_object_get_data (GObject     *object,
  * g_object_set_data:
  * @object: #GObject containing the associations.
  * @key: name of the key
- * @data: data to associate with that key
+ * @data: (nullable): data to associate with that key
  *
  * Each object carries around a table of associations from
  * strings to pointers.  This function lets you set an association.
@@ -3669,7 +3670,7 @@ g_object_dup_data (GObject        *object,
  * @oldval: (nullable): the old value to compare against
  * @newval: (nullable): the new value
  * @destroy: (nullable): a destroy notify for the new value
- * @old_destroy: (nullable): destroy notify for the existing value
+ * @old_destroy: (out) (optional): destroy notify for the existing value
  *
  * Compares the user data for the key @key on @object with
  * @oldval, and if they are the same, replaces @oldval with
@@ -3711,8 +3712,8 @@ g_object_replace_data (GObject        *object,
  * g_object_set_data_full: (skip)
  * @object: #GObject containing the associations
  * @key: name of the key
- * @data: data to associate with that key
+ * @data: (nullable): data to associate with that key
- * @destroy: function to call when the association is destroyed
+ * @destroy: (nullable): function to call when the association is destroyed
  *
  * Like g_object_set_data() except it adds notification
  * for when the association is destroyed, either by setting it
@@ -3741,7 +3742,8 @@ g_object_set_data_full (GObject       *object,
  * Remove a specified datum from the object's data associations,
  * without invoking the association's destroy handler.
  *
- * Returns: (transfer full): the data if found, or %NULL if no such data exists.
+ * Returns: (transfer full) (nullable): the data if found, or %NULL
+ *          if no such data exists.
  */
 gpointer
 g_object_steal_data (GObject     *object,